The principal objective will be to obtain new information concerning utilization of essential amino acids from foods and in crystalline form and integrate it with that already accumulated so that protein requirements can be expressed in terms of constituent essential amino acids and amino acid requirements can be interpreted in relation to their existence in dietary protein. The results will be important not only in feeding population groups throughout the world and effectively distributing available protein sources but also in treating certain medical conditions. Healthy young men and women will consume diets containing a limited amount of protein from dietary sources with or without crystalline amino acids; the latter will be administered alone in some cases. Dietary sources of protein, especially those that are consumed as staple foods, will comprise as high a proportion as possible of the diet, but free amino acids will be administered individually or together to increase flexibility. Absolute amounts and relative proportions of certain essential amino acids will be modified to ascertain interaction between quantity and proportion. The combinations to be tested will be based on data already available in this laboratory concerning utilization of amino acids from proteins and in the free form. Nitrogen retention and plasma amino acids will be the principal criteria used to evaluate these compounds.